This is another major sins that can make a person to become a kaafir.
Allaah says (interpretation of the meaning):
“Then, there has succeeded them a posterity who have given up As-Salaat (the prayers) [i.e. made their Salaat (prayers) to be lost, either by not offering them or by not offering them perfectly or by not offering them in their proper fixed times] and have followed lusts. So they will be thrown in Hell. Except those who repent…” [Maryam 19:59-60]
And the Prophet (blessings and peace of Allah be upon him) said:
“What stands between a person and kufr is giving up prayer” (narrated by Muslim).
In another narration, “The covenant that stands between us and them is the prayer; whoever gives it up has becomes a kaafir.”
Allah also says concerning the sinners: “‘What has caused you to enter Hell? ’They will say: ‘We were not of those who used to offer their Salat (prayers)’” [al-Muddathhir: 42-43].
The primary reason for which they were admitted to Hell is that they failed to offer the prayer; abandoning the prayer constitutes kufr – Allah forbid – and puts a person beyond the pale of Islam, whether he neglects it because he denies that it is obligatory, or he neglects it even though he acknowledges that it is obligatory, except in the case of one who forgets.
Shaykh Ibn ‘Uthaymeen said: The fast of one who does not pray is not valid and is not accepted, because the one who does not pray is a kaafir and an apostate, because of the verse:
“But if they repent [by rejecting Shirk (polytheism) and accept Islamic Monotheism], perform As-Salaah (Iqaamat-as-Salaah) and give Zakaah, then they are your brethren in religion” [al-Tawbah 9:11]
…Abide by the truth even though your mind deserts you.